Conventionally, crosslinked polyethylene (PEX) foam for the insulation of tubes is made in subsequent steps by extruding a sheet of crosslinked polyethylene, expanding this sheet by chemical means (e.g. azodicarbonamide blowing agent) to make a closed-cell foam sheet, then wrapping it around the tube by forming tools and welding the ends. However, because of material memory shape, welded foam sheets exhibit irregularity in their shape when wrapped around tubes. The PEX sheet has a tendency to return to its original flat shape and thus cannot stay perfectly round when wrapped around the tube. This leads to uneven foam thickness around the tube, as well as air gaps on the inside that may lead to loss of insulation and formation of condensation. Moreover, the production process involves several steps of slitting the sheets to size, corrugating the internal area of the film for better wrapping, as well as chamfering the edges of the foam prior to welding, which result in significant volume of material scrap and lower production speeds. Because the PEX material cannot be readily recycled in the same process, the scrap has a high handling cost.
In another conventional method, a non-crosslinked polyethylene material is expended by physical means (e.g. introduction of gas) in tubular foam. However, the non-crosslinked material has a lower strength than a crosslinked material and its operating temperature is about 80° C., after which it starts to gradually soften and eventually melt. This makes non-crosslinked polyethylene foam inappropriate for applications such as air-conditioning systems where the temperature of the refrigeration medium may reach up to 115° C. in its super-heated vapour phase.